Separation of lead and zinc



April 1962 T. R. A. DAVEY SEPARATION OF LEAD AND ZINC Filed 001;. 13,.1958 3,03l,2% EPARATION OF LEAD AND ZINC Thomas Ronald Albert Davey,Avonmouth, England, as-

signor to Metallurgical Processes Limited, Nassau, Bahamas, and TheNational. Smelling Company Limited, London, England, carrying onbusiness together in the Bahamas under the name and style ofMetallurgical Development Company Filed Oct. 13, 1953, Ser. No. 766,785Claims priority, application Great Britain Get. 18, 1957 4 Claims. (Cl.75-88) This invention relates to the separation of lead and zinc frommixtures thereof.

A continuous process is known for removing zinc from lead by vacuumdistillation, which produces metallic zinc containing a little lead fromlead which has been desilverized by treatment with zinc. The zincproduced by this distillation is designed for re-use in thedesilverizing of further lead bullion. In this process, zincife'rouslead is distilled in a vacuum chamber to produce zinc vapour whichcondenses on a core of solid zinc and runs down into a collecting pool.The lead content of this metallic zinc is sutiiciently high to restrictthe number of commercial purposes to which it can be put.

It is known that zinc of low lead content can be produced fromlead-containing zinc by a process of distillation with refluxing atatmospheric pressure. In a known process, the impure zinc is volatilizedand the vapour is passed through a refluxing column containing a largenumber (generally greater than twenty) of trays for condensation andre-evaporation, and zinc that is substantially free from lead iscondensed-from the vapour leaving the top of the refluxing column.

The present invention consists of a method of separating pure zinc fromzinc of low lead content in which the lead-containing zinc is distilledunder vacuum at temperatures of about 45 to 500 C., whereby the vapourobtained is practically free from lead. Under vacuum therefore the zinc(containing about 1% lead) can be distilled, and without the use of anyrefluxing column, zinc that is substantially free from lead can beobtained by condensing the zinc vapour so produced.

The present invention further consists of a process in whichzinc-containing lead is distilled under vacuum and the lead contaminatedzinc vapour is condensed in a circulated stream of molten zinc, which isat a lower temperature than the zinc-containing lead, zinc vapour beingsimultaneously evaporated from the circulated stream of molten zinc andcondensed in a zone that is at a lower temperature than the circulatedstream of molten zinc.

In one form of the present invention, zinc-containing lead, at atemperature above the melting point of zinc, flows through an outerannular evaporation zone of an evacuated vessel, a stream of moltenlead-containing zinc at a lower temperature and still above the meltingpoint of zinc, is circulated through an intermediate annular zinc-vaporrefining zone of the evacuated vessel, and centrally disposed coolingmeans are provided in an innermost annular condensation zone of theevacuated vessel. By this means some of the zinc is evaporated from thezinc-containing lead, and is condensed in the stream of molten zinc andcondensed in a zone that is at a lower zone. Zinc vapour issimultaneously evaporated from the stream of molten lead-containing zincon the inner sides of the intermediate annular zone and solid zinccondenses and accumulates on the cooling zone to a point at which itstemperature rises to the melting point of zinc and thereafter the zinccondensed here runs oii as molten zinc, the amount of molten zinc thuscondensed being approximately equal to the amount of zinc evaporatedfrom the zinc-containing lead. The circulated stream of moltenlead-containing zinc is maintained at a temperature higher 2 than themelting point of zinc but lower than that of the zinc-containing lead,and lead accumulates in this leadcontaining zinc until it becomessaturated with lead, and zinc-saturated lead separates from a zone ofthe circulatory system outside the evacuated vessel.

The invention will be further described with reference to theaccompanying drawing.

FIGURE 1 is a sectional elevation of one form of distillation apparatusincorporating the improvement.

FIGURE 2 is a section on the line IIII of FIGURE 1 and,

FIGURE 3 is an enlarged detail of FIGURE 1.

Lead containing about 1% zinc is supplied to the chamber l through apipe 2 situated just below the surface of the molten lead in the annularwell 3 at a temperature of about 560 C. The lead in this well builds upbehind a rectangular flange 4 welded to the top of an outer annularspreading wall 5. This flange has holes 6 through which lead seepsevenly around the perimeter of the spreading wall 5. This Wall has anoutwardly divergent lip 7 over which the lead flows in a thin streamcoating the inside of the wall 5 with a layer of lead. The evaporatingconditions in the chamber, with a pressure of less than 10 mm..mercury,cause the zinc in this lead to progressively evaporate out into theouter zone 8 of the chamber, as the lead flows down the wall 5 into atrough 9, at the bottom of which a pipe 10 leads to a bath of lead inthe pct 11 from which it runs continuously or can be tapped from time totime; the length of the pipe 10 above the lead in pct 11 is greater thanthe height of a barometric column of lead. The zinc vapour passesinwards towards an intermediate annular system of inclined shelves oneabove the other, which face alternately outwards 12 and inwards 13 withconnecting rods 12a and 13a (FIGURE 3). These shelves maybe constructedof refractory material or enamelled steel resistant to corrosion byzinc. These shelves are supplied with a continuous stream of molten zincat a temperature of about 450 to 500 C. After circulating over theshelves the molten zinc falls into an annular trough 14a connected by apipe 14b to a bath 14, from which a pump 15 sends it up the pipe 16 andinto an annular trough 17, from which it overflows to cascade over theshelves again. The lead which is collected by the zinc separates out ofthe zinc in the bath l4 and can be taken off at 18.

The surfaces of the outwardly facing shelves 12, over which the zinc isflowing, are exposed to the incoming zinc vapor, containing less than 1%lead, coming from the relatively hot zinc vapor in zone 8, so that thezinc vapor condenses on shelves 12, and the latent heat of condensationof this zinc causes the temperature of the liquid zinc to rise bybetween 5 C. and C. From each of the shelves 12, the liquid zinc thenflows on to one of the inwardly facing shelves 13, and, owing to thelower pressure of zinc vapor prevailing in the inner zone 19, the zincre-evaporates from shelves 13, the latent heat of evaporation thusabstracted causing the temperature of the liquid zinc flowing over theseshelves 13 to fall by approximately the same amount as it rose overshelves 12. This re-evaporated zinc vapor contains only 0.01% or lesslead. The lower zinc pressure in zone 19 prevails because the zinc vaporcondenses on the zinc collar 24). This vapour condenses on the innermostzinc collar 26* which builds up during the course of the process as asolid deposit on the condenser 21 with cooling pipe 2142 until thetemperature on the outside is sufficient to cause the zinc to remainliquid and drip down off the collar. The molten condensed zinc flowsdown pipe 25, the lower end of which is submerged in molten zinccontained in pot 26, the length of pipe 25 being greater than the heightof a barometric column of zinc. Any zinc vapour which enters thecondenser 21 is prevented from forming a solid deposit by theinstallation of a radiant heater 22. The cold trap 23 is removable inorder to clean out any deposit which may form on its water cooled walls.No zinc must be allowed to build up in the vacuum pipe 24.

Various modifications may be made within the scope of the invention.

I claim:

1. In the process for separating and recovering zinc from lead of lowzinc content in which molten zinc-containing lead at a temperature abovethe melting point of zinc is introduced under vacuum into an evaporationzone to evaporate therefrom zinc vapors containing a small quantity oflead, and in which these lead-contaminated zinc vapors are condensed toliquid metal on a body of solidified zinc in a condensation zonemaintained at a lower temperaturethan that of the evaporation zone, theliquid metal condensate being collected and recovered, the improvementwhich comprises the provision of an intermediate zinc-vapor refiningzone disposed between said evaporation and condensation zones, saidzinc-vapor refining zone having a circulating stream of molten zincflowing therethrough at a temperature between that of the evaporationzone and that of the condensation zone, the lead-contaminated zincvapors from the evaporation zone condensing in the circulating stream ofmolten zinc and zinc vapor contaminated with a lesser amount of leadsimultaneously evaporating from said circulating stream of molten zincwithout supplying additional heat to the intermediate zinc-vaporrefining zone, the zinc vapor evaporated from the refining zone beingcondensed to relatively pure liquid zinc metal on the body of solidifiedzinc in the condensation zone.

2. In apparatus for separating zinc from lead of low zinc contentcomprising an evacuated vessel within which is provided an outermostannular spreading tube over which the molten zinc-containing lead flowsin an even layer at a temperature above the melting point of zinc, andan innermost centrally disposed cooling means on which zinc vaporsevaporated at the spreading tube are condensed, the improvement whichcomprises the provision of an intermediate annular zinc-vapor refiningmeans disposed between said outermost spreading tube and said centrallydisposed cooling means, said zinc-vapor refining means comprising asystem of inclined trays disposed one above the other and facingalternately outwards and inwards, and means for maintaining a flow ofmolten zinc over said trays at a temperature above the melting point ofzinc but below the temperature of the molten zinccontaining lead flowingover the spreading tube.

3. Apparatus according to claim 2 having collecting means for collectingmolten zinc running off from the surface of solidified zinc condensed onthe centrally disposed cooling means.

4. Apparatus according to claim 2 having collecting means for collectinglead-containing molten zinc that runs off the lower end of the annularzinc-vapor refining means, and means outside the evacuated vessel forrecovering zinc-saturated lead which separates from said lead-containingzinc.

References Gated in the file of this patent UNITED STATES PATENTS2,312,811 Gentil Mar. 2, 1943 2,349,409 Davis May 23, 1944 2,478,594Queneau Aug. 9, 1949 2,720,456 Davey Oct. 11, 1955 2,823,111 Davey eta1. Feb. 11, 1958 2,939,783 Lundevall June 7, 1960 FOREIGN PATENTS711,954 Great Britain July 14, 1954 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent N0. 3,031,296 April 24, 1962 ThomasRonald Albert Davey It is hereby certified that error appears in theabove numbered patent requiring correction and that the said LettersPatent should read as corrected below.

Column 1, lines 62 and 63, for molten zinc and condensed in a zone thatis at a lower zone" read molten lead-containing zinc in the intermediateannular zone Signed and sealed this 14th day of August 1962.

(SEAL) Attest:

DAVID L. LADD ERNEST W. SWIDER I Commissioner of Patents AttestingOfficer

1. IN THE PROCESS FOR SEPARATING AND RECOVERING ZINC FROM LEAD OF LOW ZINC CONTENT IN WHICH MOLTEN ZINC-CONTAINING LEAD AT A TEMPERATURE ABOVE THE MELTING POINT OF ZINC IS INTRODUCED UNDER VACUUM INTO AN EVAPORATION ZONE TO EVAPORATE THEREFROM ZINC VPORS CONTAINING A SMALL QUANTITY OF LEAD, AND IN WHICH THESE LEAD-CONTAMINATED ZINC VAPORS ARE CONDENSED TO LIQUID METAL ON A BODY OF SOLIDIFIED ZINC IN A CONDENSATION ZONE MAINTAINED AT A LOWER TEMPERATURE THAN THAT OF THE EVAPORATION ZONE, THE LIQUID METAL CONDENSATE BEING COLECTED AND RECOVERED, THE IMPROVEMENT WHICH COMPRISES THE PROVISION OF AN INTERMEDIATE ZINE-VAPOR REFINING ZOEN DISPOSED BETWEEN SAID EVAPORATION AND CONDENSATION ZONES, SAID ZINC-VAPOR REFINING ZONE HAVING A CIRCULATING STREAM OF MOLTEN ZINC FLOWING THERETHROUGH AT A TEMPERATURE BETWEEN THAT OF THE EVAPORATION ZONE AND THAT OF THE CONDENSATION ZONE, THE LEAD-CONTAMINATED ZINC VAPORS FROM THE EVAPORATION ZONE CONDENSING IN THE CIRCULATNG STREAM OF MOLTEN ZINC AND ZINC VAPOR CONTAMINATED FROM SAID CIRCULATING STREAM SIMULTANEOUSLY EVAPORATING FROM SAID CIRCULATING STREAM OF MOLTEN ZINC WITHOUT SUPPLYING ADDITIONAL HEAT TO THE INTERMEDIATE ZINC-VAPOR REFINING ZONE, THE ZINC VAPOR EVAPORATED FROM THE REFINING ZONE BEING CONDENSED TO RELATIVELY PURE LIQUID ZINC METAL ON THE BODY OF SOLIDIFIED ZINC IN THE CONDENSATION ZONE. 